1. Field of the Invention
The present invention relates to a navigation apparatus and a navigation-related information display method that can suitably be applied to freely portable navigation devices.
2. Description of the Related Art
Known portable navigation devices (to be referred to as PNDs hereinafter) are designed to acquire data necessary for pinpointing the current geographical position (latitude and longitude) of a vehicle from at least three global positioning system (GPS) satellites and computationally determine the position of the vehicle by analyzing the data.
PNDs are also designed to be able to computationally determine the altitude of the vehicle in addition to the current geographical position (latitude and longitude) of the vehicle by acquiring and analyzing data necessary for pinpointing the current geographical position from three or more than three GSP satellites.
Then, the PND displays the current geographical position of the vehicle on a map, and after searching for available routes from the starting point to the destination of the vehicle, it navigates the user to the destination by way of the best available route.
Meanwhile, the PND requires orbit data including satellite orbit information from the GPS satellites from which it can receive signals to pinpoint the current geographical position of the vehicle. Orbit data are information that is updated every two hours and has an effective service life about four hours after the update.
Therefore, when the power source of a PND is turned off for more than four hours, it has to acquire orbit data anew from each of the GPS satellites. Since orbit data are transmitted from each GPS satellite at a cycle period of about 30 seconds, the PND desirably be able to secure a communication environment that allows it to receive orbit data continuously for at least 30 seconds.
While the PND can desirably secure a good communication environment that allows it to receive orbit data transmitted from each GPS satellite continuously for at least 30 seconds, it cannot more often than not secure such a good communication environment probably because it is running or, if it is at a stop, it is shadowed by a building.
In such a situation, the PND needs to wait until it can acquire a complete set of orbit data in the next cycle period because it cannot acquire a complete set of orbit data in the current cycle period. If the PND cannot acquire a complete set of orbit data in the next cycle period, it then has to wait until the cycle period after the next in order to acquire a complete set of orbit data.
In actuality, since the PND has to acquire orbit data from three or more than three GPS satellites, it may sometime has to spend several minutes until it can acquire a complete set of orbit data from each of the three or more than three GPS satellites.
Then, when the power source of the PND is turned on after having been turned off for more than four hours or when the PND is in a communication environment where it cannot receive signals from GPS satellites for more than four hours, the user is required to wait for several minutes until he or she can pinpoint the current geographical position of the vehicle. Then, the user may have to feel uneasy during the several minutes.
Satellite information display apparatus have been proposed to show the user the progress of the operation of acquiring orbit data from GPS satellites in the form of a numerical value or a bar graph while the PND is actually acquiring orbit data from the GPS satellites so as to relieve the user from uneasiness (refer to, e.g., Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 2002-181915).
GPS receivers adapted to display not the progress of the operation of acquiring orbit data but the signal reception level from a GPS satellite have also been proposed (refer to, e.g., Patent Document 2: Japanese Patent Publication No. 3700882).